1. Place fish in a bowl, pour over lime juice and marinate in the refrigerator, tossing occasionally, for 3 hours or until fish is opaque. Drain off half the lime juice, then add tomatoes, chillies, oregano and oil. Toss and chill for 1 hour longer.
2. Before serving, stand at room temperature for 20 minutes, then scatter with onion, olives and coriander (cilanto).